Many modern communication systems rely on spacecraft which are used as transponders. Mobile cellular communication systems have become of increasing importance, providing mobile users the security of being able to seek aid in case of trouble, allowing dispatching of delivery and other vehicles with little wasted time, and the like. Present cellular communication systems use terrestrial transmitters, such as towers, to define each cell of the system, so that the extent of a particular cellular communication system is limited by the region over which the towers are distributed. Many parts of the world are relatively inaccessible, or, as in the case of the ocean, do not lend themselves to location of a plurality of dispersed cellular sites.
In these regions of the world, spacecraft-based communication systems may be preferable to terrestrial-based systems. It is desirable that a spacecraft cellular communications system adhere, insofar as possible, to the standards which are common to terrestrial systems, and in particular to such systems as the GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS system (GSM), which is in use in Europe.
The GSM system is a cellular communications system which communicates with user terminals by means of electromagnetic transmissions from, and receptions of such electromagnetic signals at, base stations, fixed sites or towers spaced across the countryside. The term "user terminal" for purposes of this patent application includes mobile user terminals, and also includes hand-held and fixed user terminals, but not gateways. The GSM system is described in detail in the text The GSM System for Mobile Communications, subtitled A Comprehensive Overview of the European Digital Cellular System, authored by Michel Mouly and Marie-Bernadette Pautet, and published in 1992 by the authors, at 4, rue Elisee Reclus, F-91120 Palaiseau, France. Another text describing the GSM system is Mobile Radio Communications, by Raymond Steele, published 1992 by Pentech Press, London, ISBN 0-7273-1406-8. Each base station of the GSM system includes transmitter and receiver arrangements, and communicates with user terminals by way of signals in a bandwidth of 50 MHz, centered on 900 Mhz., and also by way of signals having a bandwidth of 150 Mhz centered on 1800 Mhz.
A cellular communication system should provide one or more control channels for allowing a user terminal to initially synchronize to the system, and to initiate communications with the overall network. Each base station, fixed site, or tower continually transmits network synchronization information (SCH) and network-specific information (BCCH), which a user terminal uses to synchronize to the appropriate network at initial turn-on of the user terminal. The GSM system provides a channel denominated "Random Access Channel" or RACH. In GSM, the RACH channel is used for initial synchronization of the network to the user terminal.
This invention relates to cellular communications systems, and more particularly to such systems which provide coverage between terrestrial terminals in a region by way of a spacecraft, where some of the terrestrial terminals may be mobile terminals, and some may be gateways which link the cellular system with a terrestrial network such as a public switched telephone network (PSTN).
A salient feature of a spacecraft communication satellite is that all of the electromagnetic transmissions to the user terminals originate from one, or possibly a few, spacecraft. Consequently, the spacecraft communication antenna must form a plurality of beams, each of which is directed toward a different portion of the underlying target region, so as to divide the target area into cells. The cells defined by the beams will generally overlap, so that a user communication terminal may be located in one of the beams, or in the overlap region between two beams, in which case communication between the user communication terminal and the spacecraft is accomplished over one of the beams, generally that one of the beams which provides the greatest gain or signal power to the user terminal. Operation of spacecraft communication systems may be accomplished in many ways, among which is Time-Division Multiple Access, (TDMA), among which are those systems described, for example, in conjunction with U.S. Pat. Nos. 4,641,304, issued Feb. 3, 1987, and 4,688,213, issued Aug. 18, 1987, both in the name of Raychaudhuri. Spacecraft time-division multiple access (TDMA) communication systems are controlled by a controller which synchronizes the transmissions to account for propagation delay between the terrestrial terminals and the spacecraft, as is well known to those skilled in the art of time division multiple access systems. The TDMA control information, whether generated on the ground or at the spacecraft, is ultimately transmitted from the spacecraft to each of the user terminals. Consequently, some types of control signals must be transmitted continuously over each of the beams in order to reach all of the potential users of the system. More specifically, since a terrestrial terminal may begin operation at any random moment, the control signals must be present at all times in order to allow the terrestrial terminal to begin its transmissions or reception (come into time and control synchronism with the communication system) with the least delay.
When the spacecraft is providing cellular service over a large land mass, many cellular beams may be required. In one embodiment of the invention, the number of separate spot beams is one hundred and forty. As mentioned above, each beam carries control signals. These signals include frequency and time information, broadcast messages, paging messages, and the like. Some of these control signals, such as synchronization signals, are a prerequisite for any other reception, and so may be considered to be most important. When the user communication terminal is synchronized, it is capable of receiving other signals, such as paging signals.
Communication spacecraft are ordinarily powered by electricity derived from solar panels. Because the spacecraft may occasionally go into eclipse, the spacecraft commonly includes rechargeable batteries and control arrangements for recharging the batteries when the power available from the solar panels exceeds the power consumed by the spacecraft payload. When a large number of cellular beams are produced by the antenna, a correspondingly large number of control signals must be transmitted from the spacecraft. When one hundred and forty beams are transmitted, one hundred and forty control signals must be transmitted. When the power available from the solar panels is divided between the information and data transmission channels of the spacecraft, the power available to the synchronization and paging signals may be at a level such that a user communication terminal in an open-air location may respond, but a similar terminal located in a building may not respond, due to attenuation of electromagnetic signals by the building.
As illustrated in FIG. 1, spacecraft 12 includes a transmit antenna 12at which takes the form, when deployed, of a parabolic reflector 12atr and a feed array 12atf. Feed array 12atf is mounted on the spacecraft body 12b at a location near the focus of the parabolic reflector 12atr. Similarly, a receive antenna 12ar includes a deployed reflector 12arr in conjunction with a feed array 12arf. In a preferred embodiment of the invention, the feed arrays include an array of feed horns, as described below. Gimbals designated 12gt and 12gr are mounted at the junctures of spacecraft body 12b with reflector supports 12gts and 12gtr, to allow the reflectors to be moved relative to the feed arrays in order to control the beam direction. A C-band antenna 72 is provided for communication with gateways of a cellular communication system.
FIG. 2 illustrates the layout of the horn apertures of feed horn arrangement 12atf of FIG. 1. In FIG. 2, a map of a portion of Asia is superposed on some of the circles representing apertures, distorted to appear as it would from a spacecraft to the East of the Asian coast. More particularly, Asia, together with its principal islands is designated generally as 110, 112 represents India, 114 represents the combination of Vietnam, Cambodia, and Thailand, and 116 represents the island and mainland portions of Malaysia. Some of the islands of Indonesia are represented as 118. New Guinea is illustrated as 120, and Taiwan (Formosa) by 122. The Korean peninsula is 124, and the Japanese islands are represented as 126. The circles, some of which are designated 130, represent the apertures of the various feed horns of the feed array 12atf of transmit antenna 12at of FIG. 1. Not all of the feed horn apertures are illustrated, because there are eighty-eight feed horn apertures, and illustrating them all would make the illustration difficult to interpret. For the most part, the peripheral horns of the array have been illustrated, together with a line, which is illustrated by the arrows 132, of horns across the region being served. However, it will be understood that the entire continent of Asia, and its offshore islands out as far as the Philippines, are served by spot beams originating from the eighty-eight feed horn apertures which are illustrated, in part, in FIG. 2. More particularly, the feed horn array 12atf of FIG. 1 may be represented by the outline of FIG. 2, completely filled in by circles. It should be noted that the circles of FIG. 2 do not represent the spot beam footprints themselves, but may roughly be conceived of as being a version of the footprints which each horn itself would form if it were energized independently, without a beamformer.
FIG. 3 is a more detailed illustration of antenna feed 12atf of FIG. 1. As illustrated in FIG. 3, the feed array is made up of a plurality of individual feed horns or cups 310, each of which has hexagonal peripheral walls, for close stacking of the horns in a region illustrated by dash line 312. Dash line 312 outlines a feed-horn region shaped much like the coverage region of FIG. 2. FIG. 4 illustrates one feed antenna 310 of FIG. 3, in the form of an open-ended horn or cup 408. Each feed horn 408, as illustrated in FIG. 4, includes six electrically conductive sides 410a, 410b, 410c, 410d, 410e, and 410f, extending orthogonally above an electrically conductive base or bottom 412. The upper edges of the sides 410a, 410b, 410c, 410d, 410e, and 410f together define an aperture plane 408p, better seen in FIG. 5a. The horn or cup 408 of FIG. 4 is fed by a conventional crossed dipole illustrated as 420, set in the center of the horn or cup, and supported above the bottom 412 by a combination support and balun 430, well-known in the art. As known to those skilled in the antenna arts, crossed dipole 420 includes a first dipole 420V with two colinear or coaxial elements designated 421 and 422, and a second dipole 420H with two colinear or coaxial elements designated 423 and 424. The balun aspect of support/balun 430 is provided by a pair of slots 432, 434, each having a length of about one-fifth wavelength (.lambda./5), or more exactly 0.198 .lambda., at the operating frequency, which divide the upper end of the outer shell of support/balun 430 into two portions, designated 430a and 430b. It should be emphasized that, while the dipoles are designated with V and H suffixes, they are not necessarily oriented in vertical and horizontal directions, as such may have little or no meaning in the context of operation in space. A center conductor 436 is connected by a strap 438 to the near portion 430b of the shell. In the arrangement of FIG. 4, the elements 421, 422, 423, and 424 of the two crossed dipoles 420V and 420H lie in the same plane 420p (see FIG. 5b), and their axial centerlines lie about one-quarter wavelength, or more exactly 0.249 .lambda., from the floor or bottom 412. The lengths of the elements of the two dipoles 420V and 420H are made slightly different, so that a quadrature phase shift is introduced between the two dipoles, resulting in generation of circular, or at least elliptical, polarization.
It should be emphasized that antennas are reciprocal transducer devices, which have the same characteristics in both transmission and reception, at least as to impedance at the "feed" point and as to antenna beam pattern and gain. Thus, there is no essential difference between an antenna when used for transmission and reception. However, for historical reasons, the connection to an antenna is termed a "feed" point regardless of whether the antenna is for transmission or for reception. Thus, the term "generation," when referring to the circular or elliptical polarization, applies regardless of whether the antenna is transmitting or receiving circularly or elliptically polarized signals. Also, an antenna seldom transmits or generates exactly circular polarization, it will almost always have some ellipticity. Similarly, even a perfectly circularly polarized signal, if such could be made, transmitted toward a real or practical "circularly" polarized receiving antenna would result in reception of signals which would not be equal in both principal planes.
There is a tradeoff between the power available for transmission over each antenna beam and the total power which is the solar panels 12s1 and 12s2 of FIG. 1 must produce. The total power which the spacecraft solar panels can produce is limited by the efficiency of the panels, the useful luminous flux in the region of the panels, and the pointing accuracy with which the panels can be oriented to receive the flux. With time, the solar panels may become degraded, as particles penetrate portions of the panels, and the surface of the panels becomes opaque or reflective. In a particular communication system, the communication spacecraft antenna was constructed, and a need for an additional 0.7 dB of effective radiated power or margin manifested itself after the design of the feed antenna. Whatever changes might be required to achieve the required increase in effective radiated power could not add significant weight to the already-designed structure. Since 0.7 dB corresponds to an increase in power of about 18%, the additional margin could not be achieved simply by increasing the transmitted power, because this would have required a corresponding increase in the area of the solar panels, which would have significantly increased the weight.
Attempts were made to achieve the gain increase by modifying the individual horn or cup antennas 310 of the feed arrays 12atf and 12arf by placing disk-shaped directors before the cup, but this was not successful.
Higher effective radiated power is desired from a feed array.